This invention relates to a diagnostic test method for a motor vehicle catalytic converter, and more particularly to a method for assessing the catalyst warm-up rate.
Catalytic converters are commonly installed in the exhaust gas stream of motor vehicle engines to reduce tailpipe emissions. Once the temperature of the catalyst has risen to an ideal level (about 400-700xc2x0 C.), the conversion efficiency is optimized by maintaining the engine air/fuel ratio in a specified range, typically with the aid of exhaust gas oxygen sensors located upstream and downstream of the converter. However, when the temperature of the catalyst is below the ideal level, the conversion efficiency is significantly degraded. Accordingly, a significant measure of the conversion efficiency is the rate at which the catalyst warms up following engine start-up, and it is therefore important to be able to assess the catalyst warm-up rate periodically during the life of the vehicle. Although this objective could be achieved with one or more temperature sensors placed in or near the converter, it is more desirable from a cost and reliability standpoint to use existing sensor information.
The present invention is directed to an improved method of assessing the catalyst warm-up rate of a motor vehicle catalytic converter based on the responses of fast warm-up exhaust gas sensors located in the exhaust gas upstream and downstream of the catalytic converter. The method essentially measures an oxygen storage characteristic of the catalyst that ideally increases as the catalyst temperature rises to its ideal operating temperature. According to a first embodiment of this invention, a ratio of switching frequency between the upstream and downstream exhaust gas sensors is periodically computed and compared to a threshold during catalyst warm-up. According to a second embodiment, the response times of the exhaust gas sensors to a known transition of the engine air/fuel ratio are detected to form a measure of the oxygen storage characteristic that is compared to a threshold. In each case, the threshold varies as a function of cumulative airflow through the engine to compensate for different engine airflow levels.